Each year nearly 1 million Americans suffer an acute myocardial infarction, approximately 20 percent of these individuals will die. Recent evidence has revealed that acute thrombosis plays an important role in most myocardial infarctions. In fact, it is estimated that acute thrombosis is the primary pathophysiological mechanism in 80-90% of acute transmural infarctions. Not surprisingly, recognition of the important role of thrombi and thus of platelet aggregation in myocardial infarction has intensified efforts to develop safe and effective antithrombotic agents.
A thrombus is an aggregation of blood factors, primarily platelets and fibrin with entrapment of other formed elements of the blood. Thrombi can also consist of primarily platelet aggregates. Thrombi are typically formed in order to prevent excessive bleeding from injured blood vessels. Thrombi are typically formed in the following manner.
The vascular endothelium serves as a barrier between the blood borne platelets which continually circulate throughout the body and the proaggregatory subendothelial components, which are primarily collagen. In addition to serving as a physical barrier, the cell membranes of the endothelial lining contain negatively charged components which serve to create an electrostatic repulsion between the platelets and the lining of the vessels. Trauma to the blood vessel will disrupt this endothelial lining and allow the platelets to come in contact with the underlying collagen and fibronectin. This causes the platelets to adhere to the subendothelial surface. This initial adherence causes the release, from these platelets, of a number of chemicals such as adenosine diphosphate, serotonin, and thromboxane A.sub.2, all of which have a proaggregatory effect upon the initial platelet aggregate or plug and stimulate other circulating platelets to adhere to this newly formed plug. The additional adherence of these platelets stimulate the further release of these proaggregatory chemicals, which causes further growth of the platelet plug. Thus a self-perpetuating cycle is initiated which promotes the growth of the plug.
In addition to adhering to the injured vascular wall and forming aggregates, activated platelets accelerate the generation of thrombin which acts to convert the plasma protein, fibrinogen, into fibrin, thereby stabilizing the thrombus and promoting its growth. Prior to the conversion of fibrinogen into fibrin, a sequence of enzymatic conversions take place on the platelet surface which ultimately leads to the formation of fibrin. Both the negatively charged phospholipids on the platelet surface and calcium are essential for the maximal activation of Factor X. Once Factor X is activated, prothrombin is converted to thrombin which cleaves fibrinogen into fibrin and activates Factor XIII. This Factor catalyzes the crosslinking reaction of fibrin which stabilizes the platelet mass. In addition, thrombin is a powerful platelet activator and will act to perpetuate the process.
Thus once the platelets come in contact with the subendothelial surface, a reaction is initiated in which a number of positive feedback control systems act to produce a thrombus which blocks off the affected vasculature. The entire process (i.e. platelet aggregation, fibrin generation, and polymerization) is referred to as hemostasis and is important in the prevention of excessive bleeding from the wound.
Although the formation of thrombi is desirable in a bleeding vessel, it is pathological in an intact vessel. Thrombi occur in intact vessels due to minor alterations in the endothelial cell surface or injuries that result in the disruption of the endothelial linings. Even relatively minor alterations can allow the platelets to come in contact with collagen and initiate the process described above. These minor alterations occur from a variety of causes. These causes include stasis, (i.e. decreased movement of blood in the cardiac chambers or blood vessels) which induces damage due to lack of oxygen and reduces the shear forces that ordinarily discourage platelet interaction. Another cause is the damage which the process of atherosclerosis inflicts upon the endothelial linings. Endothelial linings are known to be disrupted at the site of atherosclerotic lesion.
Thus, a significant amount of research has been focused on finding drugs which will prevent the platelets from undergoing aggregation due to these minor alterations which are commonly found on the endothelial linings. Part of the research has been directed at exploring what effect could be achieved by administering an antagonist of serotonin, one of the proaggregatory substances which is released when the platelets initially begin to aggregate. Although serotonin is a relatively weak proaggregatory factor, it has been discovered that serotonin has a synergistic effect upon the primary proaggregatory clotting factor, ADP. Thus serotonin amplifies the proaggregatory effect of ADP.
Ketanserin is a serotonin antagonist. It reacts at the 5HT.sub.2 receptor. Bush et al reported this compound was extremely effective in preventing thrombus formation in canine models which have been designed to screen for this activity. Drug Development Research, Vol. 7, pages 319-340 (1986).